The disclosures herein relate generally to flexible leaflet prosthetic heart valves and more particularly to wire stents used to reinforce such valves. Wire stents used in prosthetic heart valves are normally symmetrical in geometry. There are many known examples of such stents.
In U.S. Pat. No. 4,343,048, a stent for a cardiac valve comprises a base ring having metal legs projecting therefrom in a generally axial direction, each leg being flexible in such a manner that, when the stent has a valve installed therein and the valve is under pressure such as when operating in the heart, each respective leg can resiliently deform over substantially its whole axial length to take up strain in the valve without impairing its performance.
U.S. Pat. No. 4,501,030 discloses a prosthetic heart valve including a frame having a plurality of commissure supports, a plurality of resilient supports, and a plurality of valve leaflets. The valve leaflets are attached to the resilient supports, and the resilient supports lie radially outwardly of the commissure supports, respectively. When in use, the valve is subjected to forces which are used to clamp the valve leaflets between the resilient supports and the commissure supports to augment whatever other leaflet attachment techniques may be used.
U.S. Pat. No. 5,545,215 discloses a frame to be placed as an external support of a biological valved conduit containing three leaflets. This external frame, made of biocompatible metal or plastic, is sutured to the outer surface of the valved conduit made of biological or biocompatible membrane or sigmoid valve root in order to maintain its natural geometry. The frame has a general cylindrical configuration, circular as viewed from above and below. From a side view however, both upper and lower ends of the cylinder present three convex curvatures joined at equidistant points of the circumference. These upper and lower curves are joined by three vertical struts, so that three large saddle shaped paraboloid gaps result. The frame is a wire-like structure.
U.S. Pat. No. 4,626,255 discloses a heart valve prosthesis having a supporting frame with a circular cross-section, which is covered with a dacron fabric. On one end face, the dacron fabric is arranged to form a suture ring. The fixed aortic valve of a kangaroo is attached inside the frame and sutured to the dacron fabric.
Insert molding is a less costly method of attaching a stent to a valve body that many other methods, such as sewing and dip casting. This is especially true in high volume manufacturing. There are tight form control tolerances associated with insert molding processes for molding stents into polymer valve bodies. Despite these tolerances, the stent must fit within the cavity of the mold and remain stationary. Otherwise, the stent and mold can be damaged. Insert molding, then, is benefitted by a stent that 1) has simple features that can be held securely by the mold, and 2) has features which facilitate accurate locating of the stent in the mold. In fact, all valves, regardless of their manufacturing method, benefit from this second point, i.e. accurate location. Accurate location improves manufacturing repeatability and inspection accuracy. These lead to a more reliable product with smaller unit variability and lower manufacturing costs.
Accurate and repeatable location of a part requires defining datums. Accurate location is needed for manufacturing, assembly and inspection. A three dimensional, curvilinear part, such as a heart valve stent, complicates the method of defining these datums. One widely used method of inspection uses 6 stops on 3 mutually perpendicular datum planes to locate the part. This method, called 3-2-1 location is described in many engineering texts including xe2x80x9cFundamentals of Tool Design,xe2x80x9d 2nd Edition, Hoffman (Ed.), Society of Manufacturing Engineers, 1984 pp. 142-158, 170. Stops are used to define the first, usually horizontal, plane, and to locate the part against a vertical plane. Any additional stops, for example, a fourth stop on the horizontal plane, will over constrain the part and require deforming it to touch all stops. Plane on plane contact is another example of over constraint. Over constraining a part in manufacturing or inspection jeopardizes accuracy and repeatability.
The two configurations of stent, described above, do not lend themselves to accurate location. The stents with planar bottoms or horizontal grooves cannot be located reliably on a reference plane. A special locating block with three vertical projections is needed. None of the designs have features designed to be located against the vertical datum planes. The stents could be located in a V-block, but there is no feature to control rotation around the central axis. Parts must be located and inspected using visual alignment, which is time consuming, operator dependent, and coarse unless expensive optical equipment is used.
To make matters worse, many of the fabrication methods used in stents make it difficult to control form. Most designs contain an edge or surface that is curving in at least two directions. Metal bending (wireforms, rolled flat patterns, and drawn flat patterns) will exhibit springback and warping from residual stresses. Welding and crimping over constrain free ends. Injection molded plastics are subject to warping from shrink and relieved thermal stresses. Conventional machining gives rise to residual stresses that can warp parts.
Therefore, what is needed is a stent that provides for the tight form control tolerances of the insert molding process. Summary
One embodiment, accordingly, provides a stent having features which permit the stent to be held easily and securely, and which facilitate accurate locating for insert molding a stent in a polymer valve. To this end, a stent includes a stent member having a plurality of post members formed therein. Each post member is connected to an adjacent post member by an interconnecting portion. A plurality of leg members extend from the stent member.
A principal advantage of this embodiment is that it provides for a stent to be located more accurately and held more securely than previously known stents.